The present invention relates to a local communication system among terminal equipment in a user's premises which are connected to a local passive bus and, in particular to the improvement of a system having an ISDN (Intergrated Services Digital Network) basic-access user-network interface as defined in the I.400 series of CCITT recommendations. In such a system, the invention provides a circuit-switched type of local communication capability among ISDN terminal equipment (which means terminal equipment defined in the CCITT recommendations).
Configurations, many kinds of characteristics, and signal formats at the ISDN user-network interface were standardized as the I.400 series of recommendations by CCITT in 1984. The configurations of a standardized reference interface are as follows. Referring to FIG. 1, an ISDN user-network interface comprises a local switch (LS) 13 and a network termination unit (NT) 11 which terminates a digital subscriber line 14 extended from the LS. The NT is installed in the user's premises. A plurality of terminal equipment (TEs) 21, 22, . . . , 28 are connected through corresponding sockets 31, 32, . . . , 38 to a local passive bus (PB) 12 of a 4-wire metallic cable which is extended from the NT. Two wires 12a of the passive bus 12 are used for the signal transmission in the direction from the NT towards the TEs 21, 22, . . . , 28, and the remaining two wires 12b are used for signal transmission in the direction from the TEs 21, 22, . . . , 28 towards the NT.
FIG. 2 illustrates frame structures for signal transmission on the passive bus 12a and 12b. One frame on each bus is composed of 48 bits (corresponding to 250 .mu.s). Each of frames on the passive bus 12a has the same structure, which are repeatedly transmitted from the NT. Likewise, each of the frames on the passive bus 12b has the same structure. These frames are repeatedly transmitted from each TE. The frame for each direction contains bits of the two B-channels (B1-channel and B2-channel, each having a bit rate of 64 kbps) and bits of the D-channel (having a bit rate of 16 kbps) and are indicated as characters B1, B2 and D, respectively, in FIG. 2. The B1-channel and B2-channel are used for transmission of a circuit-switched type of communication information. The D-channel is used for out-of-band transfer of a control signal for establishment or release of the B1-channel and B2-channel between individual TEs and the NT, as well as for transfer of a packet type of communication information. One frame contains 16 of B1-bits, 16 of B2-bits and 4 of D-bits. On the passive bus 12a, E-bits for an E-channel are transmitted in one frame in addition to bits for the B1-channel, B2-channel and D-channel. Each E-bit has a copy of the D-bit on the passive bus 12b (TE.fwdarw.NT) which has lastly been received transmitting the E-bit. The E-bits, which are monitored by each terminal equipment, are used for avoiding an access collision on the passive bus 12 to ensure that, even in cases where two or more terminals attempt to access the D-channel simultaneously, one terminal will always be successful in completing transmission of its information. Furthermore, one frame on the passive bus 12a contains, in addition to the bits for the channels mentioned above, a framing bit (F) 46, a bit (A) 47 used for activation of the TEs, an auxiliary framing bit (F.sub.A) 44, a bit (N) 45 set to a binary value N=FHD A, spare bits (S1, S2) 41, 42 and DC balancing bits (L). On the other hand, one frame on the passive bus 12b (TE.fwdarw.NT) further contains a framing bit (F) 51, an auxiliary framing bit (F.sub.A) 52 and DC local balancing bits (L). In FIG. 2, dots attached to the local balancing bits (L) demarcate those parts of the frame, each part consisting of the bit following the last L bit through the L bit considered, that are independently DC-balanced.
AMI (Alternative Mark Inversion) codes with 100% pulsewidths are used as the transmission codes on the passive bus 12. Coding is performed in such a way that a binary one is represented by no pulse, whereas a binary zero is represented by a positive or negative pulse. In general, the binary zero bit has the polarity opposite to that of the binary zero bit just prior to the bit. The frame synchronization at the terminal equipment (TE) 21, 22, . . . , 28 is established by using a violation between the L-bit 50 on the passive bus 12a (which is always a negative pulse of binary zero) which follows the F-bit 46 (which is always a positive pulse of binary zero) and a binary zero bit which first occurs after the L-bit 50. This violation on the passive bus 12a is performed in such a way that the first binary zero bit following the L-bit 50, which should generally be the positive pulse, is intentionally made a negative pulse to indicate that the negative pulse prior to the pulse whose polarity has been intentionally inverted is the L-bit 50, as shown in FIG. 2. The violation can also be established by using the auxiliary framing bit (F.sub.A) 44, even when all the bits from the B1-bit 43 following the L-bit 50 to the A-bit 47 are binary ones. This violation satisfies the CCITT recommendation that on the passive bus 12a, there should always be a violation at the 14th bit or sooner from the framing bit (F) 46. Each of the TEs 21, 22, . . . , 28, detecting the F-bit 46 on the passive bus 12a, begins to send the F-bit 51 after two bits offset with respect to the F-bit 46 of the frame delivered from the network termination unit NT. Also in this case, the frame alignment in the direction TEs towards NT is established by using the violation between the L-bit 57 (which is always a negative pulse of binary zero) following the F-bit 51 and a binary zero bit which first occurs after the L-bit 57. When all the B1-bits 54 to 55 and the D-bit 56, which are between the F-bit 51 and the F.sub.A -bit 52, are binary ones, the violation can be brought out by using the F.sub.A -bit 52. Therefore, this violation satisfies the CCITT recommendation that on the passive bus 12b, there should always be a violation at the 13th bit or sooner from the framing bit (F.sub.A) 51.
In the frame structures of FIG. 2, a L-bit is a DC balancing bit for keeping a zero DC component of the region between the bit following the previous L-bit and the present L-bit. For instance, the L-bit 48 becomes a binary zero pulse of positive polarity when there exists one bit of negative DC component in the range from the B1-bit 43 to the D-bit 49. On the other hand, the L-bit 48 becomes a binary one when there exists no DC component in that range. The L-bit 57 is always a negative pulse of binary zero as described above, because the F-bit 51 is always a positive pulse of binary zero. FIG. 2 also illustrates possible electric levels of each bit. For example, the F-bit 46 is always a positive pulse and the B1-bit 43 can become either a negative pulse or no pulse.
However, the prior user-network interface mentioned above has the following disadvantage. In the configuration of the prior art, it is desirable to enable local communication among the terminals (TEs) 21, 22, . . . , 28 in the user's premises. According to the prior system, such a communication among terminals (referred to as local or inner communication hereinafter) can be performed only by a switching function of the local switch (LS) 13, because the network termination unit (NT) has no switching function for the two B-channels, allowing circuit-switched type local communication. Consequently, the prior art has the disadvantage that the local communication loads can not be realized without aid of the network or local switch.